Characterisation of the morphology of 2-D particle aggregates in different electrolyte concentrations in an ultrasound trap

An ultrasound standing wave trap in which growth of two-dimensional aggregates of suspended particles can be optically monitored has recently been developed. The technique is employed here to study the influence of [CaCl2] in the range 2-20 mM on growth of aggregates of 25 mum latex spheres. The perimeter fractal dimension for closely-packed hexagonally ordered aggregates formed by weakly interacting particles in deionised water was 1.14. The fractal dimension increased with increasing salt concentration to reach 1.58 in 20 mM CaCl2 where the aggregate resulting from high collision efficiency particle interactions was highly dendritic. The [CaCl2] at which the aggregate morphology 'crossed over' from being essentially closely packed to showing dendritic organisation was about 6 mM. Two analysis techniques, fast Fourier transform (FFT) and void analysis, that are routinely employed for characterising order or 'openness' of 2-D structures were applied to the aggregate images. The Fast Fourier Transformation method allowed the detection of the first signs of loss of the internal aggregate order at low salt molarities (2 and 4 mM). An increase in void area with increasing salt concentration was suggested at concentrations (8-20 mM) where the sensitivity of the fractal dimension to change was low. (C) 2004 Elsevier B.V. All rights reserved.